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Custom Functions

XPath 3.0 has advanced support for custom functions.

error-handling.html:
(1) 
   let $f := function ($v) {$v * 2} return $f(3)

(2)
   let
   $process :=function($v as xs:double, 
                                 $f as function(xs:double) as xs:double) 
                                as xs:double
                                {$f($v)},
   $mult2 := function ($n as xs:double) as xs:double
                  {$n*2}
   return $process(3,$mult2)

(3)
   let
   $compose:=function($f as function(xs:double) as xs:double,
                                 $g as function(xs:double) as xs:double) 
                                 as function(xs:double) as xs:double
                                 {function ($x as xs:double)
                                               {$g($f($x))}},
   $mult2 := function($x as xs:double) {$x *2},
   $mult3 := function($x as xs:double) {$x *3}
   return $compose($mult2,$mult3)(1)

(4)
   let $plus := function($x as xs:integer, 
                                 $y as xs:integer) 
                                 as xs:integer
                                 {$x+$y}
   return $plus(?,?)(2,4)

(5)
   let $plus := function ($m as xs:integer) 
                                  as (function(xs:integer) as xs:integer)
                                  {function ($n as xs:integer)
                                                {$m + $n}}
   return $plus(2)(4)

(6)
   let $s := function ($n as xs:integer, 
                              $f as function(xs:integer, 
                                  function()) as xs:integer) as xs:integer
                             {if ($n<1) then 0 else $n+$f($n - 1,$f)},
   $sum := function($n as xs:integer)
                            {$s($n,$s)}
   return $sum(3)

(1)	a simple function
(2)	passing a function to a function
(3)	functions composition
(4)	partial function application
(5)	function closure
(6)	recursive function (not supported by all)